565 codec color xform support: fix colortable / incomplete image behavior

This fixes a bug that was exposed when I added color space
support for 565 decodes.

Before this CL, we would sometimes fill incomplete images with
R and B swapped.

This fixes that issue.  Part of the fix is the decision to do 565
xforms when building the color table (then just swizzle to 565),
rather than do them per pixel after swizzling.

Bug: skia:
Change-Id: I09e1ec75aba09a4e288015ea746465d0c3f7d59f
Reviewed-on: https://skia-review.googlesource.com/11137
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
5 files changed